Optic relay for use in television

ABSTRACT

An optical relay device with an insulating plate which is ferroelectric below its Curie temperature. The plate is scanned by an electron beam. The plane of polarization of light incident on the plate is variably rotated in dependence upon the electric field created by means of the interaction between the electron beam and a signal voltage applied to the plate, due to the Pockels effect. The temperature of the plate is stabilized in the proximity of its Curie temperature. This stabilizing device uses a capacitor as a temperature sensing element. The dielectric of the capacitor is formed by a material having a Curie temperature differing from that of the plate by between 1* and 20*.

United States Patent Donjon et al. 1 Jan. 25, 1972 54 ()PTIC RELAY FORUSE IN 2,983,824 5/1961 Weeks et al. ..350/l50 3,015,693 1/1962Volberget al..... 350/150 TELEVISION 3.396.305 8/1968 Buddecke et al....350/150 [72] Inventors: Jacques Donjon, Yerres; Auguste 3.5205897/1970 Angel et al "350/150 Raymond Le Pape, Vitry Chatillon; GerardJoseph Marcel Marie, Ll'ltiy les Primary Examiner-- Robert L. GriffinRoses, all of France Assistant Examiner- Donald E. Stout Au F k R. T f73 Assignee: us. Philips Corporation, New York, N.Y. [22] Filed: Dec. 2,1969 l ABSTRACT 21 y 3 1 4 3 An optical relay device with an insulatingplate which is ferroelectric below its Curie temperature. The plate isscanned by an electron beam. The plane of polarization of light inl lFm'eign pp Dam cident on the plate is variably rotated in dependenceupon the 20 1968 Francem "179505 electric field created by means of theinteraction between the Dec. 20 1968 Fran 179504 electron beam and asignal voltage applied to the plate, due to Febv 5 1 France 69061 1 thePockels effect. The temperature of the plate is stabilized in theproximity of its Curie temperature. This stabilizing device [52] CL H178/75 BD 250/199 350/150 uses a capacitor as a temperature sensingelement. The dielec- [51] lnLCL 4 V V I I I I V H'04n 5/38 tric of thecapacitor is formed by a material having a Curie 5 Field of Search N 17154 BD 7 5 D; 250/199; temperature differing from that Of lhfi plate bybetween i and References and 6 Claims, 13 Drawing Figures UNITEDSTATESPATENTS 2,411,155 11/1946 Gorn ..350/l5(l 2 10 u. 12 32 1s 1," 8 jINFRARED I a} SUPPRESSOR BEAM DEFLECTOR 22 SlGNAL RECEIVING gbi 2.: LCIRCUIT VOLTAGE 5U PP LY PATENTEDmzsrsrz 3531931 M111 8 INFRAR DSUPPRESSOR BEAM DEFLECTOR 26 ELECTRON agYgGIFTRECEIVING GUN (62 a F lg.1

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INVENTORfi JACQUES DONJON U USTE R. LE PAPE E ARD J- M. M'ARIE OPTICRELAY FOR USE IN TELEVISION The invention relates to a device having anoptic relay, particularly for use in television. The relay comprises aplate of electrically insulating material consisting of an acid saltwhich becomes ferroelectric below the Curie temperature thereof. Thesalt is enriched with deuterium to increase the Curie temperaturethereof. The plate rotates the plane of polarization of lighttransmitted by a polarizer in accordance with a variable electric fieldwhich is applied by means ofa control electrode, so that it appearssubstantially parallel to the direction of propagation of said lightacross the plate. An analyzer transmits a selected component of thelight originating from the plate. Means are provided for scanning asurface of said plate by an electron beam. An anode receives thesecondary electrons produced by the electron beam. A temperature controldevice for stabilizing the temperature of the plate at a value in theproximity of the Curie temperature of the plate is provided. Thetemperature control device comprising as a temperature-determiningelement a capacity, the capacity of which is measured as a measure ofthetemperature.

In the picture tube of a television receiver the electron beam usuallyfulfills the following three fundamental functions:

a. the beam supplies the energy to be converted into light (thelight-transmitting power of the tube hence is always lower than thepower transferred by the beam).

b. The beam scans the surface of the picture;

c. The beam transmits the video information.

With respect to functions (b) and (c), the power of the beam and hencethe brightness of the picture cannot be increased to such an extent aswould be necessary for projection on a large screen.

It has therefore been suggested to separate these functions and to havethe function (a) fulfilled, for example, by an arc lamp and thefunctions (b) and (c) by a so called optic relay." Various types of suchrelays have been designed. The most frequently used relay(Eidophore) isheavy, bulky and hard to actuate. Another relay has been proposed byRissmann and Vosahlo (Untersuchungen zur Lichtsteuerung undBildschriebung mit Hilfe elektrooptischer Einkristalle," .lenar.Iahrbuch 1960, first volume p. 228). In this case a crystal is usedwhich has an electro-optic effect, the so-called Pockels" effect. Acrystal of KH PO. has proved suitable. This material will hereinafter bereferred to as KDP.

In so far as necessary, this effect can briefly be explained as follows:when the electrically insulating crystal is exposed to an electric fieldparallel to its crystal axis c (the three crystal axes a, b and cconstitute a trihedron of three rectangles; in this case the axis cforms the optical axis) the index of refraction of this crystal forlight rays in the c-direction with linear polarization in the plane abdepends upon the direction of polarization. If X and Y denote thebisectors of the axes a and b, and if the parameters of the crystal withrespect to these various directions are represented by the lettersdenoted for these directions, it can be said that the diagram of theindices in the plane ab becomes an ellipse with the axes X and Y insteadof a circle and that the difference nx-ny is proportional to the appliedelectric field. From this it follows that if the incident light rays arepolarized parallel to the axis a, the intensity of the light I whichtraverses an output polarizer is I=I sinkV if the direction ofpolarization of this polarizer is parallel to the axis b, and is I=I,,coskV if the said direction is parallel to the axis a, while I, is equalto the incident light intensity, if no parasitic absorption occurs,wherein V is the electric potential difference between the two facesofthe crystal and K is a coefficient which depends upon the crystallinematerial used.

For the last-mentioned optic relay a thin monocrystalline plate of KDPis used the thickness of which extends parallel to the axis 0, saidplate being provided between two polarizers. In order to obtain aprojected picture by means of a lamp with this device, it is sufficient,as described above, to apply an electric field parallel to the axis andto cause the value of the field to correspond at any point with thebrightness of the corresponding point of the picture to be obtained. Forthis purpose, an electron beam from an electron gun is caused to scanthe plate by means of conventional deflection members so that the beamfulfills the function b. The function c, here the control of theelectric field, is likewise fulfilled by the beam and that in thefollowing manner. The electrons of the beam which are incident on thesurface of the plate produce secondary electrons but with a secondaryemission coefficient smaller than I. As a result of this, negativecharges are formed at the points of the insulating plate on which thebeam was incident, which charges vary the electric field perpendicularto the plate at the relative points. The charges thus produced dependupon the accelerating voltage of the electron beam and in particular onthe anode voltage and the quantity of electricity supplied by the beam.This quantity is the product of the beam intensity and the duration ofthe passage of the beam over the relative point of the plate. The term"point" in this case has the meaning ofan elementary plane. The videosignal can previously be used for the modulation of one of these fourquantities. in the relay described, either the anode voltage or the beamintensity can be modulated but only the latter possibility is found tobe realizable. This possibility, however, has also several drawbacks.For example, the negative charge produced on the plate is not a linearfunction of the beam intensity: a further important drawback is that itis necessary for the variation of the picture that said charge isdissipated at least partly between two successive pictures. Thisdissipation involves flicker of the picture seen by the spectator theeffect of which usually is reduced only by a complication of thescanning system (interlacing). The dissipation furthermore has for itsresult that the transparency is always low. When a KDP crystal is used,it is necessary in order to dissipate the charges within less thanone-tenth ofa second to operate at the ambient temperature whichinvolves important variations of the potential of the screen of a fewkV., as a result of which the focusing of the electron beam is seriouslyhampered.

It is already known to avoid the said drawbacks oy a device of the typementioned in the first paragraph. This device is described in U.S. Pat.No. 3,520,589, the contents of which are herein incorporated byreference. In the target plate described in this application to bescanned by the electron beam and showing the Pockels effect, atemperature is used in the proximity of the Curie temperature thereof.This is possible in that the target plate consists of a salt of the KB?type, for example, a double acid phosphate or arsenate of potassium,rubidium or caesium which is enriched with deuterium, as a result ofwhich enrichment the Curie temperature is considerably increased. Thedielectric constant 5 reaches a very high value, so that, in order toobtain an adequate modulation of the light, small control voltages (V)can be applied across the target plate by means of a control electrode,since the Pockels effect is proportional to the product 6 V. It has beenproposed to control the temperature by means of a temperature controldevice which comprises as a temperature-deter mining element a capacitorthe capacitance of which is measured as a measure of the temperature,the dielectric of said capacitor being formed by a plate cut out of thesame mate rial as the said target plate. This has the advantage that themeasured capacity is proportional to the dielectric constant of thecrystal and hence proportional to the electro-optic sensitivity which isto be stabilized. However, it is difficult to approach the optimumoperating temperature, since the optimum operating temperature lies atthe temperature at which the dielectric constant of the target plate andhence also of the capacitor has a maximum.

it is the object of the invention to avoid this drawback and to providea device which comprises an efficaceous control device which is simpleof construction and with which the optimum operating temperature canalso be adjusted.

According to the invention, in a device of the type mentioned in thefirst paragraph the dielectric of the capacitor is formed by a materialthe Curie temperature of which differs from that of the said plate by afew degrees. Thus the capacity of the capacitor varies uniformly in theproximity of the optimum operating temperature. The dielectric of thecapacitor preferably has a Curie temperature which is lower than theCurie temperature of the target plate between and 20. The capacity ofthe capacitor varies in a particularly suitable manner as a function ofthe temperature in the neighborhood of the optimum operating temperatureand that in such manner that the capacity is always increasing, when thetemperature is decreasing, and the control device can be very simple.The device is of particular advantage if, according to a further aspectof the invention, it has the particular characteristic that thedielectric consists of the same acid salt as the said plate but isenriched with a lower percentage of deuterium than the material of theplate. The plate may consist, for example, ofa doubleacid phosphate orarsenate of potassium, rubidium or caesium enriched with 80 to lOOpercent deuterium and the dielectric of the capacitor may consist of thesalt of the same chemical composition enriched with from 5 to 20 percentless deuterium.

In order that the invention may be readily carried into effect,embodiment of the device according to the invention will now bedescribed in greater detail, by way of example, with reference to FIGS.I to 13 of the accompanying drawings. Corresponding components arereferred to by the same reference numerals in the various Figures.

FIG. I is a diagrammatic representation, partly in a perspective viewand partly as a block diagram, ofa known part of an embodiment in whichthe light traverses the target plate only once.

FIG. 2 shows a known part of an embodiment in which the light isreflected at a surface ofthe target plate.

FIG. 3 shows a known modification of a part of the device shown in FIG.2.

FIG. 4 is the cross-sectional view of the known vacuum tube shown inFIG. 2.

FIG. 5 is a diagrammatic cross-sectional view of the screen ofthe tubeshown in FIG. 4.

FIGS. 6 and 7 show a front elevation of two elements of the tube shownin FIG. 4.

FIG. 8 is a block diagram of an embodiment of the thermal control devicein a device according to the invention.

FIGS. 9 and 10 show modifications of the embodiment shown in FIG. 2,

FIG. It shows the beam separation polarization device shown in FIG. 10,

FIGS. 12 and 13 show modifications ofparts of FIGS. 9 and 10.

FIG. 1 diagrammatically shows members of an optic relay and the memberswhich cooperate with said relay so as to obtain a visible picture on ascreen 2 via a projection lens 4. The light is supplied by a lamp 6shown in the drawing as a filament lamp; of course, any other type maybe used. The light passes a collimator lens 8, then a space 10 whichserves for suppressing the infrared thermal rays. The optic relay ismainly constituted by a plate 12, consisting of a parallelepiped-shapedmonocrystal of KDP which contains approximately 95 percent deuteriumions calculated on the H-ions; this crystal the optical axis (0) ofwhich is perpendicular to the major planes is arranged between the twocrossed polarizers l4 and 16 the planes of polarization of which areparallel to the two other crystal axes (a and b) of the monocrystal.According to the invention, the plate 12 is kept substantially at thevalue of the Curie temperature (approximately 55 C.) by means of thetemperature control device to be described below with reference to FIG.8. Of this control device, FIG. I shows only the known thermal control18. On the left-hand surface of the plate 12 in FIG. 1, an electron beamimpinges which is denoted by a broken line and originates from anelectron gun 20. This beam periodically scans the whole effectivesurface of the plate 12 by means of deflection means 22 which iscontrolled by scanning signals of a receiver 24 which receiver receivesthe synchronization signals at the input 26 with the actual videosignal. A block 28 supplies the required direct voltage for a few of thesaid members, as well as to an anode 30. For the sake of clarity theanode is denoted by a plate parallel to the light beam; it will beobvious that this arrangement is very favorable for passing the lightbut not for receiving the secondary electrons originating from allpoints of the plate 12 on which the electron beam impinges. Therefore,in practice the anode is provided parallel to the surface of the plate12 and in the immediate proximity thereof. Since the incident electronbeam and the light beam have to traverse the anode, the anode isconstructed, for example, in the form of a grid.

FIG. I furthermore shows a thin plate 32 which is electricallyconductive and optically transparent and which in practice is formed bya thin metal layer (gold, silver, chromium) and which is surrounded byone or more metal oxide layers (SiO, SiO Bi,o,, Ag O) so as to improvethe adherence. Between this thin metal layer and the anode, the videoinformation signal is applied. It is possible to fix the potential ofthe layer at a particular value, and to supply the information signal tothe anode, but in the example described the conductive transparent layerreceives the signal so that said layer constitutes a control electrode.

The mechanism of this control may be described as follows.

When the electrons of the electron beam reach the surface of the plate,they produce, if the energy lies within the desirable limits and if theanode potential is sufficiently high, secondary electrons the number ofwhich is larger than that of the incident electrons. As a result of thisthe potential of the point of incidence is increased so that thepotential difference between the anode and the point of incidencebecomes smaller. If the electrons of the beam are incident on said pointin a sufficient number, the said potential difference becomes negativeand reaches such a value (3V, for example) that every incident electronproduces only one single secondary electron. The potential of the pointthus reaches a limit value with respect to the anode potential.Dependent upon the scanning rate, the intensity of the beam must forthat purpose be chosen to be sufficiently high. If the potential of therelative point would initially not have been lower but higher than thesaid limit value, the secondary emission would not have compensated forthe charges produced by the beam, so that said potential would havegradually reduced to the said value.

The control electrode will now be considered; if the anode potential isconstant, every passage of the electron beam, as described above, fixesthe potential of an arbitrary point A of the surface at a value V,independent of the point of incidence and of the instant of passage. Thecorresponding electric charge at the relative point, however, dependsupon the potential of the control electrode which is provided in theproximity of the other surface of the plate. It is sufficient toconsider the capacitor the dielectric of which is formed by the saidplate and the electrodes by the control electrode and the element of thesurface of incidence around the point A to see that, if V denotes thepotential of said electrode at the instant ofpassage, the charge isproportional to V,, V,,. Since the charge occurs on an electricallyinsulating surface, this remains constant till the next passage of thebeam over the same point A as well as the potential difference V,,V,,between the two surfaces of the plate at the relative point and therelative electric field which is perpendicular to the plate and thecontrol electrode. The electric field which controls the passage of thelight through the plate at the point A hence is in itself constantbetween two passages of the beam and during said passages is controlledby the video information signals. This also holds good for a furtherpoint B where the fixed potential difference when the beam passes isV,,V l being the value of the video information signal at the instant ofpas age.

The constancy of the electric field between two successive passages ofthe beam prevents flicker of the picture in such manner that, if thepicture to be reproduced is developed only very slowly, only a fewpictures per minute need be transmitted.

The above described device (FIG. 1) hence operates as follows: theelectron beam scans the plate 12; the resulting charge occurring atevery point of plate I2 depends upon the signal voltage applied to thethin plate 32 at the moment when the electron beam passes that point; inthis way an image of electric field strengths is formed on the plate 12,the electric field being directed perpendicular to the plate 12.According to the Pockels effect the plane of polarization of lightincident on the plate 12 is variably rotated in dependence upon saidfield. As a result of the crossed polarizers l4 and 16 on either side ofthe plate 12, the amount oflight reaching a point of the screen 2 isdependent on the electric field strength of the corresponding point ofthe plate 12, and in this way an image is projected on screen 2 by theprojection lens 4, corresponding with the image of electric fieldstrengths on plate 12. In consequence of the rotation of the plane ofpolarization the amplitude of the light waves passing the polarizer 16and projected on a point of the screen 2 is proportional with the sin ofthe electric field strength of the corresponding point of the plate 12,thus sin kV if V is the potential over the plate and k is a constant.The light intensity is the square of this amplitude and thus sin kV.

For clearness' sake the plate 12 in the Figure is shown perpendicular tothe light beam, only the electron beam is incident at an acute angle. Inpractice, it may be preferable, as a result of the presence of the gridin front of the screen which serves as an anode 30, to use a minimuminclination of the beam in the axis of the beam so that both beams areat an angle. It is found, however, that the KDP crystal causes a phaseshift as a result of its double refraction when a light beam passesthrough it which encloses a given angle with its optical axis c.

This phase shift is compensated by providing a crystal plate (not shown)between the polarizer, 14 and 16, the optical axis of which plate isparallel to that of the plate 12 and which shows a double refraction ofopposite signs.

In FIG. 2 the target plate (not shown) which rotates the plane ofpolarization of the light and at the surface of which the light isreflected, forms part of the screen 266 shown in the vacuum tube 50. Theincident light originates from an arc lamp 6, A capacitor C projects thepicture of the arc on a small mirror (here a totally reflecting prismR), which mirror is placed in the focus of an optic system L having afocal distance f (for example, a doublet to reduce the aberration andthe chromatism). As a result of the small dimensions of the picture ofthe source 6, the rays originating from the optic system L and incidentin the tube 50 are substantially parallel. The normal to the screen 66is slightly inclined to the axis of the beam (approximately 1) so thatthe reflected beam is focused on a plane near the mirror R prior to thebeam impinging upon the screen 2. The reciprocating paths can be variedby using the mirror R for the projection on the screen 2. The opticsystem L operates as a projection objective. The adjustment is carriedout by controlling the distance p between L and the screen 266; in thiscase l/p'+l/p must be equal to l/f, where p is the distance between theobjective L and the screen 2. In FIG. 2 the position is shown of thecrossed polarizers P, and P of the polaroid type, which are arranged inthe forward and return path, respectively.

In a known modification of this arrangement a beam separationpolarization device is used. This polarizer may comprise severaldielectric layers or be derived from the Glazebrook prism of spar as isshown in FIG. 3. This polarizer or prism replaces the mirror R of FIG. 2and in this case the polaroids P, ad P may be omitted. This has theadvantage that overlapping forward and return beams can be used as aresult of which the angle between the light beam and the screen of thetube 50 approaches 90 even more accurately. The electric field of thelight beam has a direction denoted by 267 on the left-hand side of theprism R in FIG. 3 and denoted by 268 on the right-hand side of the prismR.

The vacuum tube 50 of FIG. 2 can be constructed as shown in FIGS. 4 and5 to which reference is now made. The target plate shown in FIGS. 4 and5 is of the same type as that of FIGS. 1 and according to the inventionis kept at the value of the Curie temperature by means of thetemperature control device to be described hereinafter with reference toFIG. 8. The reference numerals l8 and BI in FIG. 4 denote a known parthereof. The light denoted by the arrows 40 (FIG. 4) which impinges uponthe plate 12 substantially perpendicularly, is reflected on the rearsurface of the plate by a mirror 42 (FIG. 5) which is electricallynonconductive. This mirror could be formed by a metal layer vapordeposited in a vacuum through the grid 30; in the example shown, themirror has a multiple dielectric and is formed by seven layersalternately of zinc sulphide and cryolite; the thickness of each layeris equal to one-fourth wavelength of the light. In order to increase thesecondary emission coefficient a cryolite layer 44 with double thicknessis added. The rear surface of the plate 12 receives the electron beamsupplied by the gun 20. This beam is accelerated by a voltage of 2,000volt between the cathode 202 with the filament 204 and the electrode206. The beam passes a gridlike electrode 30a and the gridlike anode 30before reaching the plate 12. The gridlike anode 30 receives secondaryelectrons from the plate 12. Secondary electrons not intercepted by theanode 30 are intercepted by the gridlike electrode 30a having anappropriate potential. The beam is then deflected magnetically by thefour coils 22 after focusing by means of the coil 46, so that in thecase of an intensity of 26 pa. the current density at the level of thelayer 44 is lA/sqcm. In this manner a scanning of 600x800 discretepoints on a surface area of the plate 12 of 27 mm. X 36 mm. is obtained.

The layer 44, the mirror 42 and the various other layers are providedone after the other.

As a result of the use of the mirror 42, various advantages are obtainedwith respect to the device shown in FIG. 1.

As a result of the use of the mirror 42 the light has to traverse theplate 12 two times, as a result of which with a given thickness of theplate and a given electric voltage the resulting phase shift is doubled.The analysis by the electron beam is also facilitated when said beam isincident perpendicularly. Moreover the light does not traverse the anode30, so that a better permeability is obtained.

FIG. 5 shows on the right-hand side the anode 30 formed by a grid andcovered with a receiving member for the secondary electrons originatingfrom the layer 44, under the action of the incident electrons of thebeam transmitted by the gun 20. The receiving grid consists of copper.The pitch is 50;; and the thickness is approximately lOu. The diameterof the apertures is approximately 45 so that the permeability for theincident electrons lies between 60 and percent. This grid is stretchedon an annular support 52 (FIG. 4) consisting of a coppermickel alloyhaving a coefficient of expansion which is approximately equal to thatof copper. This support has an ef fective circular passage of 40 mm.diameter and comprises a narrowed portion 54 in which a copper-nickelring can be accommodated. In order to secure the grid to the support,the ring is forced into the narrow portion 54 while the grid is takenalong, after which the ring and the support are secured together byspot-welds. After mounting the grid is thermally hardened so as toobtain a suitable mechanical stress of the grid.

The support 52 of the grid 30 comprises a gap 56 for passing theconnection wire 58 of the control grid 32 and six holes as denoted by60. The support is secured by screws 62 on a tray 64; both componentsare applied to earth potential. The depth of the tray is equal to thethickness of a disc 66, the function of which will be described below.The disc has a thickness of 8 mm. and a diameter ofS cm. In order tomaintain the space of 50p. chosen between the grid 30 and the layer 44,mica spacing members (not shown) are provided between the support 52 andthe tray 64. In front of the plate 12 is arranged the control electrode32 which must have a sufficient thickness to have a low resistance persquare which in this case is lower than 500 ohm, (the resistance persquare is measured between two parallel sides of a square with thelayer). However, the electrode must be thin so as to obtain a goodtransparency. In

the example described, the electrode is formed by a metal layer (gold,silver, chromium) coated with one or more metal oxide layers 32] and 322to improve the adherence (SiO, SiO B50 Ag,0, for example).

Reference is now made to FIG. 6 and FIG. 7.

The sensitive layer 12 has substantially the shape of a rectangle of 3X4cm. (FIG. 6). An aluminum layer 70 which leaves an effective aperture of27 mm X 36 mm. and which enables a good electrically conductiveconnection to the control electrode 32, is provided at the edges of saidrectangle on the rear surface. When the plate 12 is provided on the disc66, said layer 70 contacts an aluminum layer 72 (see FIG. 7) which isprovided in four sectors on the front surface of the disc 66. These foursectors which are separated from each other enable the electric contactbetween the layers 70 and 72 to be checked after the provision. On thelayer 72 the connection wire 58 is situated through which the videoinformation signal is supplied to the electrode 32. The thickness of theplate 12 is 0.2 mm. which is compatible with the said definition(600x800 points) in the proximity of the Curie temperature thedielectric constant is much higher in the direction of the optical axisof the crystal than in any other direction. As a result of this thelines of force of the electric field cannot deviate from the normal tothe plate, while throughout the thickness hereof the definition obtainedin the division of the charges on the layer 44 can be maintained.

It has already been noted above that in the examples described the plate12 is kept substantially at the Curie temperature by means of thecontrol device to be described with reference to FIG. 8. In theembodiments described, as shown, (only) in FIG. 4, the plate 12 is forthat purpose secured to a fluorine plate 66 having a coefficient ofexpansion approaching that of KPD provided in a tray 62 of copper whichis cooled by the hollow ring 80 which is connected to a Joule Thompsoncryostat 18, to which nitrogen under a pressure of I50 kg/sqcm. issupplied. Only the plate 81 of the cryostat is denoted in FIG. 4. Withreference to FIG. 8, the cryostat is formed by a narrow tube 82, whichends in a small aperture and is wound within an inner tube 83 having athermally insulating wall. The gas expands via said aperture, so that itis cooled and said gas in turn cools the in-flowing gas during itsescape along the narrow tube. The temperature gradually decreases. Thesupply of nitrogen from a bottle 84 is controlled by an electricallyoperated valve 86 which in itself is controlled on the basis of themeasured capacity of the capacitor 90 consisting of the plate 88 coveredwith two electrodes. The plate 88 has a diameter of from 3 to mm, and athickness of approximately 0.5 mm. It consists of KDP which is enrichedwith 5 to percent less deuterium than the plate 12 and is provided nearthe plate 12 (not shown). In the tube shown in FIG. 4 it is adhered, forexample, to the free surface of the grid support 52. The characteristicof the dielectric constant of the material of the plate 88 in accordancewith the temperature, enables the optimum operating temperature to beadjusted for the plate 12.

The other elements of the control device are: and electric oscillator 92of 2 mc./s., a capacitor 94 which forms a capacitive bridge with thecapacitor 90, an amplifier 96, a detector 98, the electromagnetic part100 of the valve 86 and finally a controllable threshold formed by apotentiometer I02 and a direct voltage generator 104.

The device according to the invention can be improved by the measureshown in FIGS. 9 to 13.

Whereas in FIG. 2 the optic system L operates as a collima tor and as aprojection objective, FIG. 9, to which reference is now made, shows thecollimator separated from the objective. The collimator is constitutedby the plane-convex lens formed by the fluorine plate 66 which alsoserves for the heat dissipa tion of the target plate 12. A beamseparation polarization device R is used. It may comprise severaldielectric layers or be derived from the Glazebrook prism of spar as isshown in the Figure. The objective 125 is arranged between the beamseparation polarization device R and the projection screen 2.

The lens can as a result be manufactured without special measures withrespect to thermal or mechanical stresses since it is avoided that theobjective serves as L in FIG. 2, as if it were placed between twocrossed polarizers, whereby each stress in the objective lens isexpressed in the appearance of parasitic light on the projection screenand results in errors in the uniformity of the brightness of the pictureand a reduction of the contrast. Since the collimator 66 is arrangedsubstantially in the objective plane, it has only an extremely weakinfluence on the operation of the objective 125. For the calculation ofthe objective 125 only the curvature of the field as a result of thecollimator need be taken into account as regards the collimator. Thelight source in FIG. 9 is of the film projection type and consists of alamp 6 having a mirror 123 in the form of an ellipsoid of revolution.

The device shown in FIG. 9 may furthermore be improved by using themeasures shown in FIG. 10. Whereas in the device shown in FIG. 9 onlyhalf of the light intensity which is incident on the prism R is used,since the surface 31 reflects only one of the polarized components ofthe light to the plane 12, the whole light intensity in the device shownin FIG. 10 is used by making use of a multiple beam separationpolarization device R, a plate 127 which shifts the phase by half awavelength, two flat mirrors 128 and 130 and one concave mirror 129.

FIG. 11 shows in detail the multiple beam separation polarization deviceR of FIG. 10 derived from the Glazebroolt type of spar and the directionof the electric vectors of the components of the incident and emanatingpolarized light. The optic axis of the spar is perpendicular to theplane of the drawing. Along the surfaces 131 and I32, three prismsequivalent to the two prism of R of FIG. 9 are combined by a glue theindex of refraction of which lies in the proximity of the extraordinaryindex of refraction of the spar (n L486) which is lower than theordinary index of refraction (n 1.658). The prism 136 allows the lightwhich passes the surface 131 to emanate. This prism 136 is equivalent toa layer with parallel surfaces for the projection beam and may consistof glass or another transparent isotropic material which is united withthe three other prism by means of a suitable glue the index ofrefraction of which may be different from the said index of refraction nAs shown in FIG. 10, the mirror 12 forms a picture of the source 6 inthe beam separation polarization device R. The component of the lightthe electric vector of which is parallel to the plane of the drawing, isreflected at the surface 131 to the collimator 66 (see FIGS. 10 and II).The component the electric vector of which is perpendicular to the planeof the drawing traverses R directly and then the plate 127. Beyond thelate 127 the electric vector is parallel to the plane of the drawing. Bymeans of the flat mirrors 128 and 130 the concave mirror forms a secondpicture of the source of the beam separation polarization device. Theconcave mirror which operates with a magnification equal to I, may bespherical. By moving the concave mirror to the right or to the left inFIG. 10 only one ofthe flat mirrors I28 and I30 is sufficient.

The multiple beam separation polarization device R may also bemanufactured entirely from glass when several dielectric layers are usedat the separating surfaces 13] and 132. In this case the directions ofthe electric vectors are the inverses of those which are shown in FIG.11.

With suitable choice of the indices of refraction the faces 13] and 132may enclose angles of 45 with the axis and, as shown in FIG. 12, when aquarter wavelength plate 127 is used which is passed two times, theprism I36 and the flat mirrors I28 and 130 may be omitted.

In the device shown in FIGS. 9 and 10 the light source 6 for the mirror123 a shadow which involves a nonuniform illumination of the targetplate and hence brightness errors on the screen 2. In order to avoidthis the devices may further be improved by using the measure shown inFIG. 13. In FIG. 13 an assembly of two mirrors 141 and 142 which areshifted from each other by a few millimeters and enclose an angle of afew degrees with each other is arranged between the mirror 121 and thebeam separation polarization device R so that the light source 6 infront of the mirror 123 is not visible when the mirror is observedthrough the beam separation polarization device by means of the mirrors141 and 142. The mirrors 141 and 142 are preferably cold mirrors," whichare transparent to infrared radiation so that heating of the beamseparation polarization device, of the target plate and of other opticalmeans used is counteracted.

In order to prevent that secondary electrons originating from the pointof the screen 266 hit by the electron beam impinge upon points which mayhave higher potentials than the point of the grid 30, this grid may beprovided, according to a known measure, very close to the screen byadhering the grid 30 to the screen after the surface of the grid hasbeen covered with an insulating layer. Furthermore, in order to preventincident electrons on the screen 266, according to a known measure, amagnetic field perpendicular to the surface of the screen is used. Thesemeasures prevent efficaciously that the secondary electrons originatingfrom the point hit by the electron beam are received by adjacent points.Upon using these measures alone, however, it remains possible thatsecondary electrons, after having covered a path of several millimetersor several centimeters, can return to all points of the screen whichhave a potential equal to that or higher than that of the screen. Inorder to remove this drawback, the last electrode of theelectron-optical lens system in the tube is preferably set up at apotential which is equal to or higher than the highest potential whichany point of the screen can reach. Since this highest potential withrespect to the potential of the grid in absolute value is equal to thepotential difference between a white" point and a black" point, apotential difference which is at least equal to the maximum peakto-peakcontrol voltage must be applied between the said last electrode and thegrid. In practice the last electrode then has a potential which isapproximately 100 to 200 volts higher than that of the grid. The lastelectrode preferably is a second grid provided in front of the firstgrid 30 at a few millimeters distance therefrom on the cathode side, towhich second grid a potential is applied which is lOO to 200 volt higherthan that of the first grid. This provides the advantage that aparticularly uniform collection of the secondary electrons is alsoobtained. Since the second grid is not placed in the focal plane of thebeam it may have wider meshes than the first grid, the advantage of agreat transparency being obtained. The second grid preferably consistsof parallel wires stretched perpendicularly to the direction of thescanning lines in order to avoid Moire phenomena. When the first gridhas a rectangular structure, the two orthogonal directions of saidstructure preferably are oriented so that they enclose angles of 45 withthe scanning lines and with the direction of the wires of the secondgrid. When the first grid has a "hexagonal" structure, the orientationthereof with respect to the scanning lines and the wires of the secondgrid is not critical. It is to be noted that the above-mentioned secondgrid can be used together with a first grid secured to the screen and amagnetic field, but, if desirable, for the sake of particularrequirements, may also serve to replace a first grid adhered to thescreen or a magnetic field.

What is claimed is:

1. An optical relay device, comprising a plate of electricallyinsulating material positioned in an optical path between a polarizerand an analyzer, said material consisting of an acid salt which isferroelectric below the Curie temperature thereof, said material beingenriched with deuterium whereby the Curie temperature thereof is higherthan in the absence of deuterium, means for applying a variable electricfield ac ross the plate with a direction parallel to the generaldirection of propogation of light in said path whereby the plane ofpolarization of the light is variably rotated in dependence upon saidfield, means for generating an electron beam, means for scanning theelectron beam across a major face of said plate, an anode for collectingsecondary electrons released from said plate by said electron beam, anda tem erature control device for stabilizing the temperature of t eplate at a value differing at most 5 from the Curie temperature of theplate, said temperature device comprising a capacitor as atemperature-determining element the capacity of which varies as afunction of the temperature, the dielectric of the capaci tor comprisinga material having a Curie temperature differing from that of the plateby between 1 and 20.

2. A device as claimed in claim 1, wherein said anode is a gridlikeanode between the major face of said plate and said scanning means, saidanode being disposed parallel to and within 1 mm. from said surface,said device further comprising means for applying a potential to saidanode, a gridlike electrode disposed parallel to said gridlike anode onthe side of said scanning means and means for applying a potential tosaid gridlike electrode having a voltage higher than the potentialapplied to said anode.

3. A device as claimed in claim 1, wherein the dielectric of thecapacitor has a Curie temperature which is between 5 and 20' lower thanthe Curie temperature of the said plate.

4. A device as claimed in claim 1, wherein in that the dielectric of thecapacitor consists of the same acid salt as the said plate but isenriched with a lower percentage of deuterium than the material of thesaid plate.

5. A device as claimed in claim 2, wherein said gridlike electrodecomprises parallel wires stretched perpendicular to the lines of thescanning.

6. A device as claimed in claim 5, wherein the gridlike electrode has awider pitch than said anode.

1. An optical relay device, comprising a plate of electricallyinsulating material positioned in an optical path between a polarizerand an analyzer, said material consisting of an acid salt which isferroelectric below the Curie temperature thereof, said material beingenriched with deuterium whereby the Curie temperature thereof is higherthan in the absence of deuterium, means for applying a variable electricfield across the plate with a direction parallel to the generaldirection of propogation of light in said path whereby the plane ofpolarization of the light is variably rotated in dependence upon saidfield, means for generating an electron beam, means for scanning theelectron beam across a major face of said plate, an anode for collectingsecondary electrons released from said plate by said electron beam, anda temperature control device for stabilizing the temperature of theplate at a value differing at most 5* from the Curie temperature of theplate, said temperature device comprising a capacitor as atemperature-determining element the capacity of which varies as afunction of the temperature, the dielectric of the capacitor comprisinga material having a Curie temperature differing from that of the plateby between 1* and 20*.
 2. A device as claimed in claim 1, wherein saidanode is a gridlike anode between the major face of said plate and saidscanning means, said anode being disposed parallel to and within 1 mm.from said surface, said device further comprising means for applying apotential to said anode, a gridlike electrode disposed parallel to saidgridlike anode on the side of said scanning means and means for applyinga potential to said gridlike electrode having a voltage higher than thepotential applied to said anode.
 3. A device as claimed in claim 1,wherein the dielectric of the capacitor has a Curie temperature which isbetween 5* and 20* lower than the Curie temperature of the said plate.4. A device as claimed in claim 1, wherein in that the dielectric of thecapacitor consists of the same acid salt as the said plate but isenriched with a lower percentage of deuterium than the material of thesaid plate.
 5. A device as claimed in claim 2, wherein said gridlikeelectrode comprises parallel wires stretched perpendicular to the linesof the scanning.
 6. A device as claimed in claim 5, wherein the gridlikeelectrode has a wider pitch than said anode.